1. Field of the Invention
This invention relates to an image recording apparatus, and more particularly to an image recording apparatus for recording a two-dimensional image on a two-dimensional recording medium by imaging thereon light emitted from a plurality of light emitting elements arranged in one direction.
2. Description of the Related Art
There has been known a line recording type image recording apparatus which comprises a line recording source comprising a number of light emitting elements linearly arranged in a main scanning direction and an erecting unit optical system comprising a refractive index distribution type lens array which images light emitted from the line recording source as an erected image in an unit magnification on a two-dimensional recording medium and in which a two-dimensional image is recorded on the two-dimensional recording medium by imaging light emitted from the respective light emitting elements of the line recording source, the intensity of light emitted from the respective light emitting elements being modulated according to the image to be recorded, through the refractive index distribution type lens array while moving the two-dimensional recording medium in a sub-scanning direction intersecting the aforesaid main scanning direction. See, for instance, U.S. patent application No. 882763(1997).
The degree of freedom of the refractive index profile of the refractive index distribution type lens array is limited for reasons of production and it is difficult to obtain a refractive index distribution type lens array having a desired resolution and a large numerical aperture. Accordingly, the part of the light emitted from the light emitting elements which does not impinge upon the refractive index distribution type lens array within the range of a predetermined numerical aperture is absorbed or reflected at the side face of the lens array and cannot be imaged on the two-dimensional recording medium. That is, a part of the light emitted from the light emitting elements cannot be used for forming an image on the two-dimensional recording medium and is emitted in vain. Especially, in a dry printer where a heat-developing film is used and a large amount of heat energy is used to heat-develop the film, there has been a demand that the light emitted from the light emitting elements is better used to reduce the power consumption and the light emitting element cost.
In view of the foregoing observations and description, the primary object of the present invention is to provide an image recording apparatus which can better use the light emitted from the light emitting elements to record the image.
In accordance with the present invention, there is provided an image recording apparatus which comprises a plurality of light emitting elements arranged in a main scanning direction and an imaging optical system which images light emitted from the light emitting elements on a two-dimensional recording medium in a linearly arranged fashion and in which a two-dimensional image is recorded on the two-dimensional recording medium by imaging light emitted from the respective light emitting elements, the intensity of light emitted from the respective light emitting elements being modulated according to the image to be recorded, through the imaging optical system while moving the two-dimensional recording medium in a sub-scanning direction relatively to the imaging optical system, wherein the improvement comprises that the imaging optical system comprises a first optical system comprising a plurality of biaxial optical elements, each having a refractive index profile in the main scanning direction and a refractive index in a direction perpendicular to the main scanning direction, arranged in the main scanning direction, and a second optical system comprising an optical element disposed on the light incident side of the first optical system or on each of the light incident side and the imaging side of the first optical system and having a refractive power to light components propagated in a direction perpendicular to the main scanning direction but no refractive power to light components propagated in the main scanning direction.
The imaging optical system may be, for instance, an optical system which images light emitted from the light emitting elements as an erected image in an unit magnification on the two-dimensional recording medium.
It is preferred that the first and second optical systems be formed integrally with each other.
The two-dimensional recording medium may be, for instance, a heat-developing film.
The light emitting element may be selected from the group consisting of an LED element, an LD element and an organic EL element.
The sub-scanning direction need not be perpendicular to the main scanning direction so long as it intersects the main scanning direction.
The two-dimensional recording medium need not be in the form of a flat sheet but may be a recording medium having a curved recording surface such as a cylindrical surface. That is, the two-dimensional image may be recorded on a curved surface as well as a flat surface.
In the image recording apparatus of the present invention, the collecting efficiency of the light components propagated in a direction perpendicular to the main scanning direction can be increased without deteriorating the resolution since the numerical aperture (NA) in the direction perpendicular to the main scanning direction can be increased by virtue of the second optical system having a refractive power to light components propagated in a direction perpendicular to the main scanning direction but no refractive power to light components propagated in the main scanning direction irrespective of limitation in the degree of freedom of the refractive index profile of the first optical system, whereby the light emitted from the light emitting elements can be better used to record the image.
When the imaging optical system is an optical system which images light emitted from the light emitting elements as an erected image in an unit magnification on the two-dimensional recording medium, the image represented by the intensities of the light emitted from the respective light emitting elements can be more precisely formed on the two-dimensional medium.
When the first and second optical systems are formed integrally with each other, the optical system holding mechanism can be simplified and the imaging optical system can be small in size. At the same time, shift of each optical system relatively to the other optical system due to vibration or the like can be suppressed, whereby the optical performance of the imaging optical system can be held constant for a long time. Further, when the light emitting element is an LED element, an LD element or an organic EL element, the image recording apparatus can be smaller in size.
The present invention is especially useful when it is applied to an image recording apparatus, where the two-dimensional recording medium is a heat-developing film requiring a large amount of optical energy.